1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an in-plane switching (IPS) mode LCD device and a fabrication method thereof that prevent a blot phenomenon, such as a chuck blot, causing an external light to be reflected.
2. Discussion of the Related Art
In light of the recent developments in various portable information display electronic devices, research is actively ongoing focusing on several types of flat panel display devices including liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, and vacuum fluorescent display (VFD) devices. In particular, LCD devices have received much attention because they are light and thin, are simple to mass produce, produce high quality images, and have a low power consumption.
An LCD device displays images by controlling a light transmittance of a liquid crystal layer with pixels arranged in a matrix form and applying a data signal according to picture information to each pixel. The LCD device is commonly driven using an active matrix (AM) method in which a switching device, such as a thin film transistor (TFT), is provided in each pixel to apply a voltage to each pixel to change the light transmittance therethrough.
There are various display modes of the LCD devices based on how liquid crystal molecules are driven. For example, a twisted nematic (TN) mode is commonly employed in LCD devices due to advantages, such as easy black and white display function, rapid response time and low driving voltage. The TN mode LCD device drives liquid crystal molecules using an electric field perpendicular to a substrate, such that a director of liquid crystal can have an angle between 0° and 90° with respect to the substrate. However, the TN mode LCD device is disadvantageous in that a viewing angle for an observer is undesirably narrow because the liquid crystal molecules are aligned vertically by the electric field perpendicular to the substrate.
To solve such a narrow viewing angle problem, a new technology, such as an in-plane switching (IPS) mode LCD device, has been proposed. In an IPS mode LCD device, when a voltage is applied to an electrode, an in-plane electric field, which is a horizontal field with respect to a substrate, is formed to align liquid crystal molecules horizontally to obtain wide viewing angle characteristics.
FIG. 1 is a plan view illustrating a portion of a thin film transistor array substrate of an in-plane switching mode liquid crystal display device according to the related art. In FIG. 1, a TFT array substrate includes a gate line 16 and a data line 17 formed thereon crossing each other, thereby defining a pixel region. In the pixel region, a plurality of common electrodes 8 for switching liquid crystal molecules and a plurality of pixel electrodes 18 are alternately disposed parallel to the data line 17 to generate in-plane electric field on the array substrate. The pixel electrodes 18 are electrically connected to each other by a pixel electrode line 18L via first contact holes 40A, and the common electrodes 8 are electrically connected to each other by a common electrode line 8L via second contact holes 40B. The pixel electrode line 18L and the common electrode line 8L are disposed parallel to the gate line 16.
A switching device, such as a thin film transistor (TFT) 20 is provided at the crossing of the gate and data lines 16 and 17. The TFT 20 includes a gate electrode 21, a semiconductor layer (not shown), a source electrode 22 and a drain electrode 23. The gate electrode 21 extends from the gate line 16, the source electrode 22 extends from the data line 17, and the drain electrode 23 extends from the pixel electrode line 18L. In particular, the gate electrode 21 receives a scanning pulse applied from the gate line 16 to turn on the TFT 20. Thus, the TFT 20 selectively transfers a data signal from the data line 16 to the pixel electrodes 18. The common electrodes 8 and the pixel electrodes 18 are formed of a transparent conductor on the same plane to enhance an aperture ratio and luminance of the LCD device. However, it lowers contrast ratio because complete black luminance cannot be obtained in a normal black mode.
In the IPS mode LCD device, various types of patterns, such as lines and electrodes, are formed by repeating a photolithography process. The photolithography process includes coating, exposing, developing, etching and removal a photoresist material. For example, after an object layer is formed on a substrate on which a pattern is desired to be formed, a photoresist material is coated. Then, the substrate is loaded to an exposing device, in which the substrate and a mask are aligned and then light is irradiated the substrate through the mask. Subsequently, the exposed photoresist film region is developed to form a certain photoresist pattern. Then, the object layer is etched into a desired pattern by using the photoresist film pattern.
If the object layer positioned on the substrate is made of a transparent film, when the substrate loaded on a chuck is exposed, light beam irradiated on the substrate transmits through both the transparent film, namely, the object layer, and the substrate, as well as the photoresist film. Then, light is reflected by the chuck made of metal to re-transmit through the photoresist film. As a result, the photoresist film positioned at the upper portion of the chuck is exposed twice by the reflected light, thereby increasing the light exposure amount. Thus, there is a difference of the light exposure amount between a region of the substrate which contacts with the chuck and a region of the substrate which does not contact with the chuck. Because of the difference in light exposure, the shape of the photoresist pattern, such as its width, differs according to each region, resulting in generation of a critical dimension (CD) deficiency with a transparent electrode formed through the photoresist pattern, which results a blot on a screen.
As a result, when the pixel electrode and the common electrode of the IPS mode LCD device are made of the transparent conductor, the width of the electrode which determines an aperture ratio of the LCD device changes according to each region on the substrate due to the exposed amount by the reflected light. Hence, a chuck blot is generated on a display screen of the LCD device.